Thanks for the response, wickidwombat.
I agree that comparison is not quite that simple; as some items do carry more weight than others, based on what is important on one's shooting needs. So, your points are well taken.
When I pull the trigger to upgrade from my crop sensor XSi to a FF, and spend the mega bucks, naturally I expect the new camera to be quite capable and useful for every situation. I do a lot of landscape, fair amount of indoors low-light, and some events photography (indoor concerts). Regarding the AF system, would you think 5D3's system is easy to understand and use, and would it be hugely advantageous over 6D's super-sensitive center point low-light AF?
Also, another question I have, which really applies to both cameras, is that since they don't have pop flash, how would you manage the highly back-lit subjects without a fill flash (that is if you don't have an external flash with you). I am a bit concerned with spot metering the main subject, which could wash-out the back ground (i.e. would lose the nice blue sky background on a sunny day).